FIBER OPTIC GYRO-BASED ATTITUDE DETERMINATION FOR HIGH- PERFORMANCE TARGET TRACKING

Transcription

1 FIBER OPTIC GYRO-BASED ATTITUDE DETERMINATION FOR HIGH- PERFORMANCE TARGET TRACKING Elias F. Solorzano University of Toronto (Space Flight Laboratory) Toronto, ON (Canada) August 10 th, th AIAA/USU Conference on Small Satellites

2 Research Motivation Earth observation (EO) market opportunities Agriculture Precision Farming Crop Health & Production Environmental Monitor Carbon Emissions Forestry Operations Humanitarian Aid Disaster Response Crisis Management Applications beyond LEO Asteroid mining Planetary exploration August 10 th, th AIAA/USU Conference on Small Satellites SSC16-VIII-1 2

3 SFL-Built Target Tracking Platforms The Next-Generation Earth Monitoring and Observation (NEMO) bus is developed specifically for ground-target tracking applications. The standard NEMO-bus structure is used for EO-specific missions such as NEMO-AM and GHGSat-D. +Y +Z +X SFL s standard NEMO-class spacecraft GHGSat-D being launched on-board a PSLV rocket on June 2016 (credit: ISRO) August 10 th, th AIAA/USU Conference on Small Satellites SSC16-VIII-1 3

4 NEMO Bus: System Overview Suitable for autonomous observations of selected ground targets. Peak tracking control error constrained below 0.3 on target-tracking mode. Specification Dimensions Max. Weight Peak C Nominal Value 20 x 20 x 40 cm 15 kg 80 W Reaction Wheels (3) Magnetorquers (3) Sun Sensors (6) Star Tracker Payload Capacity ACS Stability 1 Using MAG, FSS, and RWs 2 With Star Tracker 9 kg ~ 2 1 ~ 60 2 Optical Payload ACS design satisfies the requirements of current EO missions. Future missions may require superior pointing performance. August 10 th, th AIAA/USU Conference on Small Satellites SSC16-VIII-1 4

5 NEMO Bus: ADCS Architecture Orbit and attitude determination EKF and controller run at 1Hz. PID three-axis controller based on quaternion and rate feedback. Control modes: Inertial Pointing Align & Constrain ECEF-frame Target Tracking Other features: Gyroscopic torque cancellation. August 10 th, th AIAA/USU Conference on Small Satellites SSC16-VIII-1 5

6 Terrestrial Target Tracking Overview Spacecraft is commanded to maintain line-of-sight (LoS) with specific ground target. Only star tracker measurements (sampled at 1Hz) are used to determine satellite s attitude during the slew maneuver. May not be sufficient for future missions requiring pointing performance better than 0.3 (2-σ). High-grade fiber-optic gyro (FOG) measurements can be used to augment the star tracker. STK-generated animation of a NEMO-class satellite in target tracking August 10 th, th AIAA/USU Conference on Small Satellites SSC16-VIII-1 6

7 Fiber Optic Gyros (FOGs) High-grade miniaturized FOGs are suitable for high-rate targettracking maneuvers. High bandwidth Low noise parameters (e.g. angular random walk, bias drift, etc.). Commercially accessible ST is augmented using FOG measurements at high cadence ( 2Hz), allowing for more frequent control torque commands. Can compensate for the attitude error drift caused by invalid star tracker measurements. August 10 th, th AIAA/USU Conference on Small Satellites SSC16-VIII-1 7

8 The Challenge To design a technology that improves the current pointing performance of SFL s terrestrial target tracking spacecraft by allowing the ACS subsystem to run at frequencies above 1Hz. Proposed Solution: To implement a combination of star tracker and high-grade FOG running at high cadence. Modify our existing attitude determination algorithms to account for additional gyro states. August 10 th, th AIAA/USU Conference on Small Satellites SSC16-VIII-1 8

9 GUIDANCE & NAVIGATION August 10 th, th AIAA/USU Conference on Small Satellites SSC16-VIII-1 9

10 Trajectory Construction: Slew Maneuver Initial orientation Payload boresight alignment Imaging Constraint August 10 th, th AIAA/USU Conference on Small Satellites SSC16-VIII-1 10

11 Trajectory Construction: Orbital Parameters Position and Velocity at t k and t k+1 are obtained in the ECEF and converted into ECI coordinates to be used in the navigation model. : Target Position Vector : Satellite Position Vector : Satellite-To-Target Vector Pointing Geometry as Observed in a non-rotating ECEF Frame August 10 th, th AIAA/USU Conference on Small Satellites SSC16-VIII-1 11

12 Trajectory Construction: Desired Attitude and Rates Desired attitude, body rates and angular accelerations can be constructed based on position and velocity. Inertial-To-Body Attitude: Desired Angular Rate: Desired Angular Acceleration: Target Tracking Trajectory at Discrete Timesteps August 10 th, th AIAA/USU Conference on Small Satellites SSC16-VIII-1 12

13 ATTITUDE DETERMINATION August 10 th, th AIAA/USU Conference on Small Satellites SSC16-VIII-1 13

14 Attitude Determination: Overview State Vector Measurements Original: x = T ω bi T q bi T Interoceptive: To: x = T ω bi T q bi st g bt T g Reaction Wheel Torques Environmental Disturbances where, ω bi R 3 : spacecraft body rate q bi R 4 : attitude quaternion s g R 3 : gyro scale factor Exteroceptive: Star Tracker Quaternions FOG Angular Rates b g R 3 : gyro bias August 10 th, th AIAA/USU Conference on Small Satellites SSC16-VIII-1 14

15 Attitude Determination: State Propagation Star Tracker t ST FOG CTRL Torque Commands t k t k+1 State Propagation using FOG only Filter Initialization (ST + FOG) Notation: t ST t k t k+n : Star Tracker Exposure Time : Simulation Time August 10 th, th AIAA/USU Conference on Small Satellites SSC16-VIII-1 15

16 Attitude Determination: EKF Architecture Interoceptive Measurements Prediction Exteroceptive Measurements Correction August 10 th, th AIAA/USU Conference on Small Satellites SSC16-VIII-1 16

17 TERRESTRIAL TARGET TRACKING SIMULATIONS August 10 th, th AIAA/USU Conference on Small Satellites SSC16-VIII-1 17

18 Target Tracking Simulation: Overview 500km sun-synchronous orbit Total duration is 650 seconds Epoch: 29-August-2014 [08:32:15 UTC] Peak angular velocity of ~0.8 /s directly overhead% Control torques, disturbances and true body attitude/rates generated using SFL s high-fidelity simulator. Torques are used as interoceptive measurements in the EKF. True attitude/rates are perturbed with noise in sensor models to simulate exteroceptive measurements. Top to Bottom: Reaction wheel torques, disturbances, and body rates. August 10 th, th AIAA/USU Conference on Small Satellites SSC16-VIII-1 18

19 Attitude Determination Cases Case 1: Simulation runs at 1Hz, single star tracker sampling at 1 Hz (current method used in orbit). Star Tracker FOG Elapsed Time [sec] Case 2: Sensor fusion using FS-EKF, simulation runs at 5Hz, star tracker at 1Hz, and fiber optic gyro (FOG) at 5Hz. Star Tracker FOG Elapsed Time [sec] Case 3: Simulation runs at 5Hz, and star tracker at 1Hz (open loop approach). No FOGs. Star Tracker FOG Elapsed Time [sec] August 10 th, th AIAA/USU Conference on Small Satellites SSC16-VIII-1 19

20 Rate estimation errors using FS-EKF are within the 0.01 /s req. circle 100% of the time. Slightly better attitude estimates. Overall fusion approach offers better attitude estimates than ST-only solution. 5Hz star tracker-only open loop solution resulted in the worst estimation out of all cases. Attitude Estimation Errors (X-Y Plane) Attitude Determination: Preliminary Results RMS Attitude & Rate Estimation Errors (over 650 seconds) Error Type RMS Attitude Error RMS Rate Error [ /s] [ ] δθ x δθ y δθ z δω x δω y δω z 1Hz-ST FS-EKF Hz-OL Rate Estimation Errors (X-Y Plane) August 10 th, th AIAA/USU Conference on Small Satellites SSC16-VIII-1 20

21 Conclusion & Future Work A fiber optic gyro-based attitude estimation scheme has been proposed to allow the controller on-board NEMO-class target tracking satellites to command torques at 2Hz to improve pointing performance. Future work: The modified Kalman filter (FS-EKF) will be implemented in the loop with the controller to demonstrate that the control pointing accuracy can be reduced substantially and bounded well below 0.3 (2-σ). August 10 th, th AIAA/USU Conference on Small Satellites SSC16-VIII-1 21